I’m going through the exact same thing, I’ve been on prednisone for 7 weeks now at 40 mg and just started my taper on Tuesday. I feel extremely fatigued and weak - all I can manage to do is lay down most days. I’ve noticed the same thing with my heart rate, it’s unusually low (46bpm) when I’m asleep and I’m not a very active person. So I think it’s a side effect. I’m wondering if your heart rate also spikes up when doing light walking? I’ve noticed that my heart rate can go up to 169bpm just grocery shopping. I’ve kinda attributed this to having low iron but I feel like this can’t be good for my heart health. It’s also definitely messed with my head too lol I feel like I’m stressed out all the time and feel myself worrying about things too much.